Intermediate System to Intermediate System (ISIS) is a routing protocol designed to move information efficiently within a computer network, a group of physically connected computers or similar devices. ISIS accomplishes this by determining the best route for packets through a packet-switched network. The protocol was defined in ISO/IEC 10589:2002 as an international standard within the Open Systems Interconnection (OSI) reference design. Though originally an ISO standard, the IETF republished the protocol as an Internet Standard in RFC 1142. IS-IS has been called the de facto standard for large service provider network backbones.
Shortest Path Bridging (SPB) uses ISIS as the control protocol to transfer routing information between devices in an SPB Network acting as a transport network between access networks which may be running different protocols. In an SPB network, the ISIS Link State Database (LSDB) is used to advertise routing information. In addition to information about adjacencies with other SPB enabled devices the LSDB also includes reachability information for services outside the SPB network. Examples are—IPv4 Unicast routes, IPv4 Multicast Routes, IPv6 Unicast routes, IPv6 multicast routes, L2 and L3 virtual Service networks (VSNs), Unicast Backbone Media Access Control (BMAC) addresses etc.
When ISIS comes up, there is certain information that needs to be learned (link state information, etc.) in order to build forwarding tables. There can be a lot of churn in all the nodes as the learning of the information happens simultaneously. This in turn can result in nodes have an inconsistent view of the network. This is especially true in larger networks or networks where different routing protocols are used within the same network. It may be convenient at times for a router originating link state information to control its usage by other nodes in the network. In this case the control is in signaling other nodes as to when to use the link state information to compute paths transiting through the originating node.
One known solution is to set the overload bit field in Link State Packet (LSP) 0 to artificially create a condition where the other nodes in the ISIS network will not compute routes transiting through the advertising node. When the advertising node is ready to signal other nodes to compute transit routes through it, it will update its LSP 0 to clear the overload bit and flood the LSP. All major router vendors support the use of the overload bit to delay route computation through an advertising ISIS node.